The present invention relates to an optical coupling device for two-way optical communications over a single optical fiber and, more particularly, to an optical coupling device disposed opposite the end face of the optical fiber, through which received light output from the optical fiber is guided to light receiving means and transmission light emitted from light emitting means is launched into the fiber end face.
For two-way optical communications over a single optical fiber, it is necessary to guide at either end of the fiber transmission light emitted from a light emitting element into the optical fiber and light output therefrom into a light receiving element. A general configuration of such an optical coupling device is one that optically couples the light emitting element and the light receiving element to either end face of the optical fiber through utilization of transmission and reflection of light by means of a half mirror or prism. FIG. 1 is a diagrammatic showing of an example using a prism as the optical coupling element in combination with an optical fiber 12, a light emitting element 14 and a light receiving element 13. In this example, the optical coupling device is formed by a prism 11, through which light is transmitted and received.
The prism 11 in this example is of rectangular equilateral triangle in section. The optical fiber 12 is disposed with its one end face adjacent a first one (11a) of two planes of the prism 11 which form the right angles between them, and the light receiving element 13 is disposed with its condenser lens 17a adjacent the second plane 11b of the prism 11. Disposed outside a third plane 11c of the prism 11 which forms an oblique side is the light emitting element 14.
With such an arrangement, received light 21 output from the end face of the optical fiber 12 is launched into the prism 11 through the plane 11a, then reflected by the plane 11c to the plane 11b, and launched therefrom into the light receiving element 13. On the other hand, transmission light 22 emitted from the light emitting element 14 enters the prism 11 through the plane 11c, that is, it passes through the plane 11c to the plane 11a, thereafter being launched into the end face of the optical fiber 12.
The use of the prism 11 as described above permits favorable positioning of light emitting means (light emitting element 14) for launching the transmission light 22 into the optical fiber 12 and light receiving means (light receiving element 13) for receiving the received light output from the optical fiber 12.
With the FIG. 1 configuration, letting the diameter of the optical fiber 12 be represented by A0 and the spread diameter of the transmission light 22 on the end face of the optical fiber 12 be represented by A1, the quantity of transmission light 22 to be launched into the optical fiber 12 bears a relationship, A02/A12, to the quantity of transmission light 22 having entered the prism 11 through the plane 11c and passed through the prism 11xe2x80x94this suggests a considerably low coupling efficiency.
On the other hand, letting the diameter of the condenser lens 17a be represented by B0 and the spread diameter of the received 21 at the position of the condenser lens 17a be represented by B1, the quantity of received light 21 incident on the condenser lens 17a bears a relationship, B02/B02, to the quantity of received light 21 reflected by the plane 11c of the prism 11, which suggests a low coupling efficiency at the receiving side, too, as is the case with the transmitting side. The problem of low coupling efficiency still remains unsolved as well in the case of using the prism 11 coated over the entire area of its oblique plane 11c with a polarization reflecting film to form a polarization beam splitter.
The coupling efficiency could be increased, for example, by the placement of a condenser lens between the end face of the optical fiber and the prism 11, but the introduction of such an optical system separate of the prism 11 inevitably gives rise to a problem that the device becomes bulky and expensive accordingly.
It is therefore an object of the present invention to provide an optical coupling device for two-way optical communications which is small and inexpensive.
According to the present invention, the optical coupling device for two-way optical communications over a signal optical fiber, comprising:
a prism having a first plane containing an area opposite the end face of said optical fiber, a second plane containing an area opposite light receiving means and a third plane containing an area opposite light emitting means; and
a cylindrical member embedded in said prism in that area of said first plane opposite the end face of said optical fiber and having a refractive index larger than that of said prism;
wherein received light launched into said prism from said optical fiber is output to said light receiving element via said cylindrical member, and transmission light launched into said prism from said light emitting element is output to said optical fiber via said cylindrical member.